Extended projections induced by partial deafferentation of the medial mammillary nucleus

Abstract

Quantitative electron microscopy of degeneration has been used to study the pattern of distribution of projection fibres from the hippocampal complex to the posterior part of the medial mammillary nucleus (MMp). Two days after bilateral destruction of the fornix column almost 20% of the presynaptic terminals showed electron dense degeneration. These terminals were evenly distributed across the midline and filled the entire mediolateral and dorsoventral extent of the nucleus on both sides. Unilateral fornix lesions gave the same distribution of degeneration as bilateral lesions but with half the overall density of synapses. The overall hippocampal projection through the fornix column is formed by the coalescence of two separate fibre contingents which initially run through the dorsal fornix and the fimbria. Bilateral lesions of the dorsal fornix caused degeneration throughout the mediolateral extent of the MMp, but predominantly in its dorsal half. Bilateral fimbrial lesions gave a complementary pattern of degeneration, which filled the ventral half of the nucleus throughout its mediolateral extent. Unilateral fimbrial lesions gave a similar distribution of degeneration to bilateral lesions, although with about half the density of terminals. Lesions were placed bilaterally in either the dorsal fornix or the fimbria, and a survival time of 3–4 months allowed. This is sufficient for all the degeneration to be removed. Two days before sacrifice the remaining tract was destroyed bilaterally. In both cases the previous lesion caused the remaining tract to acquire approximately double its normal number of terminals, and to extend its distribution into the territory denervated by the first lesion. Thus, after prior fimbrial lesions the dorsal fornix fibres extended ventrally from their normal territory and established synaptic terminals virtually as far as the ventral margin of the nucleus. Conversely, after prior dorsal fornix lesions the fimbrial fibres extended their terminal distribution dorsally as far as the dorsal margin of the nucleus, although the newly acquired terminals formed in the denervated territory were neither as numerous nor as evenly distributed as those in the original fimbrial territory. This suggests that denervation induces sprouting, but that the efficiency of reclamation of the denervated postsynaptic sites is limited, probably by competition from other axonal systems in the region.